Ultrafiltration (UF) encompasses a variety of membrane filtration techniques in which hydrostatic pressure forces a liquid against a semipermeable membrane.
In blood treatment, UF generally denotes a process of removing water from blood plasma. Blood is passed on a blood side of a blood filter, and a gradient of pressure is created through the semipermeable membrane. The pressure gradient forces fluid through the pores of the membrane. The pores filter electrolytes and small and middle sized molecules (up to 20,000 to 30,000 daltons) from the blood plasma. In contrast to the plasma, the ultrafiltrate output from the filtration pores lacks the plasma proteins and cellular components of plasma. Since the concentration of small solutes is the same in the ultrafiltrate as in the plasma, fluid volume is removed without any change in the plasma concentration.
Slow Continuous Ultrafiltration (SCUF) is a continuous therapy which is designed to approximately mimic the ultrafiltration function of the kidneys. During SCUF, blood is removed from the body of a subject and passed in an extracorporeal circuit through a blood filter, where a predetermined percentage of plasma water is removed based upon a prescription. Typically, no more than 2 liters an hour of fluid is removed. The remaining blood is returned to the patient. Unlike hemodialysis, hemofiltration and hemodiafiltration, no dialysis fluid or replacement fluids are used in SCUF.
SCUF may, e.g., be employed for treatment of congestive heart failure (CFH) or other conditions leading to fluid overload in a subject. CHF is a condition that occurs when the heart becomes damaged and reduces blood flow to the organs of the body. If blood flow decreases to a certain degree, kidney function becomes impaired and results in fluid retention, abnormal hormone secretions and increased constriction of blood vessels. This results in increased workload of the heart and further decrease of the heart's pumping ability which, in turn, causes further reductions in blood flow to the kidney. It is believed that the progressively-decreasing perfusion of the kidney is the principal non-cardiac cause perpetuating the downward spiral of the so-called “Vicious Cycle” of CHF. Moreover, the fluid overload and associated clinical symptoms resulting from these physiologic changes may be seen as the predominant cause for excessive hospital admissions, terrible quality of life and overwhelming costs to the health care system due to CHF. This fluid overload can be removed by means of SCUF.
SCUF may also be used as a supplementary treatment to dialysis, such as hemodialysis. Hemodialysis is a standard technique for treating patients suffering from acute or chronic renal failure. Hemodialysis treatments are traditionally carried out about three times a week, whereby the blood is purified and the liquid balance of the patient is adjusted at a hemodialysis site in a clinic. Since the kidneys of a patient suffering for renal conditions usually do not produce any significant amounts of urine, the patient may turn up at the hemodialysis site with an excess of liquid in the body. After the hemodialysis treatment, the patient may instead have a shortage of liquid in the body. By subjecting the patient to SCUF in between the ordinary hemodialysis treatments, it is possible to equalize the liquid level over time and thereby reduce the strain on the patient's body caused by fluctuating liquid levels.
SCUF may also be used for treatment of anuric patients treated by peritoneal dialysis (PD), which may experience fluid overload.
It is generally desirable that the subjects/patients are able to perform a SCUF treatment outside of a clinical environment, e.g. at home. Specifically, there is a need for an ambulatory ultrafiltration device that enables continual, steady and smooth removal of excess fluid from the body of a subject, preferably while being worn by the subject.
WO2004/026364 discloses an ultrafiltration device adapted to be worn on a portion of a body of a patient. The device includes a blood filter including a blood inlet tube leading from a first blood vessel and blood outlet tube leading to a second blood vessel in the patient. A blood pump forces the patient's blood through the filter. Excess fluid is separated by the blood filter and drained into an excess fluid bag.
A similar device is disclosed in EP1509262, which includes a blood filter with an inlet for connection to an artery of a patient and an outlet for connection to a vein of the patient. Blood is driven to continuously flow through the extracorporeal circuit by the inherent pressure difference between the artery and the vein, and the excess fluid which is separated by the blood filter is drained into a filtrate container.
These wearable devices thus require connection of two access devices (needle catheters, etc) to the vascular system of the patient. This also means that there is a risk for severe blood loss if the venous access device is detached, since blood will be driven from the arterial access device through the blood filter and out of the detached venous access device. The risk for detachment of an access device may be elevated in a wearable device, and the resulting blood loss may be significant, especially if the device operates continuously, albeit at a low blood flow rate. The need for access to two blood vessels may also be undesirable, in particular if the device is to be installed by persons without medical training and experience, e.g. by the subject itself.
U.S. Pat. No. 7,311,689 discloses an ultrafiltration device which is not portable, let alone wearable, but which is designed to perform SCUF for removal of excessive liquid in patients suffering from CHF. In one disclosed embodiment, the device is connected to the vascular system of the patient by means of a single needle. The device comprises a bifurcated line set with a first branch for connecting the needle to a bag via a pump, and a second branch for connecting the needle to the bag via a blood filter and the pump. A valve is arranged in each of the first and second branches. The device operates in a withdrawal phase, in which the pump and the valves are operated such that blood is withdrawn via the needle into the first branch, thereby bypassing the filter, and stored in the bag. Then, in a return phase, the pump and the valves are operated such that blood is ejected from the bag into the second branch for transport back to the patient. In the return phase, ultrafiltration occurs as the blood passes the blood filter, and the resulting ultrafiltrate is collected in a bag connected to the filter. Apart from not being designed for ambulatory treatment, this device has an elevated risk for clotting and/or coagulation of the blood in the bifurcated line set, since blood is stagnant in one branch while blood is transported in the other. Furthermore, compared to the above-identified wearable devices, more blood is contained in extracorporeal circuit and is exposed to a larger surface area of foreign material. Still further, the disclosed embodiment necessitates the use and control of valves, and the additional structural complexity may lead to an increased risk of system failure.